1. Field of the Invention
The present invention relates to a charging apparatus and an image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copying machines, printers and facsimile units, images are formed as follows. By imparting electric charges to a surface of a photoreceptor on which surface a photosensitive layer is formed as an image bearing member and contains a photoconductive substance, the surface of the photoreceptor is uniformly charged. Subsequently, an electrostatic latent image corresponding to image information are formed in various image forming processes. The electrostatic latent image is developed by a toner-containing developer supplied from a developing section, so as to obtain a visible image which is then transferred onto a recording material such as paper. After that, the visible image is fixed on the recording material by heat and pressure given by a fixing roller so that an image is formed on a recording sheet.
In the image forming apparatus as described above, a charging apparatus is used for charging the surface of the photoreceptor. The charging apparatus is composed of: a discharging electrode for conducting corona discharge on the photoreceptor; a grid electrode which is an electrode disposed between the surface of the photoreceptor and the discharging electrode and to which appropriate voltage is applied, for controlling an amount of charges imparted from the discharging electrode to the surface of the photoreceptor, and thus carrying out control of charged potential of the surface of the photoreceptor; and a support member for supporting the discharging electrode and the grid electrode. Since the grid electrode is capable of nearly precisely controlling the charged potential of the surface of the photoreceptor, a charging apparatus provided with the grid electrode is becoming predominant at present.
As the discharging electrode for the charging apparatus, for example, there are used a wire electrode and a metal plate electrode having a plurality of needle portions (hereinafter referred to as “needle electrode”). Among them, the needle electrode is preferably used because of its advantages such as a less number of components, a longer operating life, less generation amount of ozone, and less frequent failures by virtue of no disconnection. The needle electrode is manufactured by etching a metal plate which is mainly made of an iron-based metal material such as stainless steel, to form a plurality of needle portions in the metal plate. The needle electrode manufactured by etching is also referred to as an etched electrode. An etched cross section of the needle electrode lacks in the smoothness. Further, a plurality of edges for conducting discharge are present at the top end of the needle portion, and shapes of the edges present at the top ends of a plurality of needle portions are not uniform. Accordingly, discharges from the respective needle portions are not uniform. As a result, it is not possible to sufficiently control the charged potential of the surface of the photoreceptor, so that the potential of the charged surface of the photoreceptor becomes uneven.
Further, the material for the needle electrode, i.e. the iron-based metal material such as stainless steel has high durability, but involves a drawback of being easily oxidized due to moisture under a high humidity circumstances and due to ozone generated by corona discharging during a charging operation. Moreover, in a long-time use of the needle electrode, a use of the needle electrode under the high humidity circumstance, a contact of the needle electrode with ozone, etc. are inevitable. Accordingly, the needle electrode made of a metal material such as stainless steel corrodes due to moisture in air or due to ozone etc., resulting in deterioration of durability thereof. In addition, there is lowered controlling performance for high voltage applied to the needle electrode for conducting the corona discharge from the needle portion, with the result that the potential of the charged surface of the photoreceptor is uneven, causing a to-be-solved problem that the surface of the photoreceptor cannot be constantly charged to a desired potential in a stable manner. Further, as in the case of the needle electrode, the wire electrode also involves a to-be-solved problem such that ozone generated by corona discharge leads rust, corrosion, or other troubles which cause the potential of the charged surface of the photoreceptor to be uneven.
In view of the foregoing problems in the charging apparatus, there has been proposed, for example, a charging apparatus including: a wire electrode extended in a shield case having one surface thereof open; and a plate grid electrode arranged between the wire electrode and the photoreceptor, the plate grid electrode which is formed by applying a nickel plated layer of about 1 μm thickness on a surface of a porous stainless steel plate and further forming thereon a gold plated layer of about 0.3 μm thickness (refer to, for example, Japanese Unexamined Patent Publication JP-A 11-40316 (1999)). In the plate grid electrode of JP-A 11-40316, the gold plated layer is formed on the porous stainless steel plate by way of the nickel plated layer so that the gold plated layer is less easily peeled off, and the corrosion resistance and the controllability of charged potential of the surface of the photoreceptor are relatively satisfactory.
However, the manufacture of the plate grid electrode requires two plating steps, i.e., nickel plating and gold plating. This poses drawbacks such as more complicated manufacturing step and increase of the cost. Further, in order to make this plate grid electrode sufficiently exhibit the preferred characteristics as described above, a thickness of the gold plated layer needs to be set at 0.3 μm or more. In addition, since the plate grid electrode is a relatively large member having a substantially the same size as the photoreceptor, the usage of gold is necessarily increased also because of the necessity for increasing the thickness of the plated layer. However, such a heavy usage of gold unnecessarily increases the cost of the charging apparatus and thus the cost of the image forming apparatus, resulting in loss of the general applicability of the image forming apparatus based on its relatively low price which is one of the advantages of the image forming apparatus. Accordingly, there has been a demand for realizing, without using an expensive material such as gold, a charging apparatus having the needle electrode and the plate grid electrode which are excellent in durability and controllability of charged potential of the surface of the photoreceptor.
Further, there has been proposed a charging apparatus including: a wire electrode; and a plate grid electrode which is obtained by forming a gold plated layer directly on a surface of a stainless steel metal plate by an electrolytic plating method using a pulse current, as in the case of JP-A 11-40316 (refer to, for example, Japanese Unexamined Patent Publication JP-A 2001-166569). Also in this plate grid electrode, the gold plated layer is less easily peeled off and, like the plate grid electrode in JP-A 11-40316, the corrosion resistance is high and the controllability of charged potential of the surface of the photoreceptor are also favorable. However, also in this plate grid electrode, a thickness of the gold metal layer needs to be set at 0.3 μm or more, it involves the same drawbacks as those in the charging apparatus of JP-A 11-40316.
On the other hand, it has also been proposed to coat the discharging electrode with gold (refer to, for example, Japanese Unexamined Patent Publication JP-A 2004-4334). The charging apparatus of JP-A 2004-4334 includes a needle electrode on a surface of which a coating layer made of gold, platinum, copper, nickel or chromium is formed by plating. In JP-A 2004-4334, a method of etching, precision pressing, or the like method is used for forming the needle electrode, but the cross section of the needle electrode obtained by the method lacks in smoothness and results in fine irregularities. Accordingly, even after applying the plating, fine irregularities on the cross section remain as they are and may disturb the balance of the corona discharging, causing the surface of the photoreceptor to be charged to uneven potential. Further, contaminants such as a toner are easily deposited on the fine irregularities. That is, the needle electrode of JP-A 2004-4334 has a drawback that contaminants such as the toner are attached thereto during a long-time use and this causes the charged surface of the photoreceptor to have further uneven potential.
As the grid electrode, for example, there are used a wire grid electrode made of stainless steel, tungsten or the like ingredient, and a porous plate grid electrode formed of a metal plate (grid substrate) of stainless steel or the like ingredient with a large number of perforations. Note that, upon manufacturing the porous plate grid electrode, etching or other methods can be adopted for making the perforations in the metal plate. The porous plate grid electrode manufactured by etching is referred to as an etching grid. Contaminants such as a toner are easily deposited on the wire grid electrode among the grid electrodes described above. Due to such deposition of the contaminants, the function for control of charged potential of the surface of the photoreceptor becomes insufficient, and this causes a problem that the potential of the charged surface of the photoreceptor is further uneven.
On the other hand, in the case of using the porous plate grid electrode which has a relatively large area compared to the wire grid electrode, the charged potential of the surface of the photoreceptor can be controlled to fall in an appropriate range and in addition, the controllability for the charged potential is decreased to a smaller extent even when the contaminants are deposited more or less. Further, the porous plate grid electrode is made of an iron-based metal material such as stainless steel as described above, and therefore exhibits high durability and does not cause troubles such as deformation even after a long-time use, with the result that the change of the controllability for the charged potential due to deformation or the like is extremely small. Accordingly, it is considered that the porous plate grid electrode can control the charged potential of the surface of the photoreceptor so as to be substantially constant for a long period of time.
The material for the porous plate grid electrode, i.e. the iron-based metal material such as stainless steel has usually high durability, but involves a drawback of being easily oxidized due to moisture under a high humidity circumstances and ozone generated by corona discharging during a charging operation. In a long-time use of the porous plate grid electrode, a use of the porous plate grid electrode under the high humidity circumstance, a contact of the porous plate grid electrode with ozone, etc. are inevitable. Accordingly, the porous plate grid electrode made of a metal material such as stainless steel corrodes due to moisture in air or due to ozone etc., thus causing a nitrogen oxide to be deposited on the surface of the porous plate grid electrode which is thereby deteriorated in durability. In addition, the ability of controlling the charged potential of the surface of the photoreceptor is decreased to an insufficient level, and with the result that the potential of the charged surface of the photoreceptor is uneven, causing a problem that the surface of the photoreceptor cannot be constantly charged to a desired potential in a stable manner.
In view of the problems of the porous plate grid electrode as described above, there has been proposed a corona charging apparatus having a grid electrode composed of a metal plate (grid substrate) made of stainless steel, and a nickel plated layer and a gold plated layer which are sequentially coated on a surface of the grid electrode (refer to, for example, Japanese Unexamined Patent Publication JP-A 11-40316 (1999)). In the grid electrode, improvement is indeed recognized to some extent in view of the corrosion resistance. However, the grid electrode involves a drawback that the gold plated layer which serves as a main factor for enhancing the corrosion resistance, is easily peeled off. Although JP-A 11-40316 discloses that the nickel plated layer is formed between the metal plate and the gold plated layer in order to prevent the gold plated layer from being peel off, the effect is not yet satisfactory.
Further, there has been proposed a charging apparatus having a grid electrode formed by applying a gold plated layer directly to a stainless steel-made metal plate (grid substrate) without interposing a nickel plated layer therebetween by an electrolytic plating method using a pulse current (refer to, for example, Japanese Unexamined Patent Publication JP-A 2001-166569). The charging apparatus is adopted in an image forming apparatus including: an organic photoreceptor on which an electrostatic latent image is formed; a charging apparatus for charging the organic photoreceptor; a developer for developing the electrostatic latent imaged formed on the organic photoreceptor into a toner image; a transfer portion for transferring the toner image onto a recording material; and a fixing device for fixing the transferred toner image on the recording material. In the charging apparatus, the gold plated layer is less easily peeled off and therefore, the corrosion resistance and the controllability of charged potential of the surface of the photoreceptor are satisfactory. By the way, in order to make this grid electrode sufficiently exhibit the preferred characteristics as described above, a thickness of the gold plated layer needs to be set at 0.3 μm or more. In addition, since the grid electrode is a relatively large member having a substantially the same size as the photoreceptor, the usage of gold is necessarily increased also because of the necessity for increasing the thickness of the plated layer. However, such a heavy usage of gold unnecessarily increases the cost of the charging apparatus and thus the cost of the image forming apparatus, resulting in loss of the general applicability of the image forming apparatus based on its relatively low price, which is one of the advantages of the image forming apparatus. Accordingly, there has been a demand for realizing, without using an expensive material such as gold, a charging apparatus having the needle electrode and the grid electrode which are excellent in durability and controllability of charged potential of the surface of the photoreceptor.